The present invention relates to a multi-chip module having a plurality of semiconductor integrated circuit elements (e.g. IC chips) and a cooling member soldered on the semiconductor integrated circuit elements.
As operation frequencies of semiconductor integrated circuit elements speed up, the heat generated from such semiconductor integrated circuit elements increases corresponding to a greater extent. The heat from such semiconductor integrated circuit elements can be reduced with air-cooling methods, air-blast cooling methods, and water-cooling methods. One of these cooling systems is selected appropriately to the heat value of the semiconductor integrated circuit elements. However, even water-cooling systems, which are the best in cooling performance, require higher efficiency of heat transmission between the back side of each of the semiconductor integrated circuit elements and the water-cooling member.
Higher efficiency heat transfer methods, such as the comb-teeth method, the heat transfer grease or the adhesive heat transfer method, and the solder-fixing method, are now under investigation. Among these heat transfer methods, the solder-fixing method with a higher heat transfer rate, which uses a low fusion point metal, is the highest cooling efficiency method. Such solder-fixing methods are disclosed in, for example, U.S. Pat. No. 5,325,265, Unexamined Published Japanese Patent Application Nos. Sho 60-253248, Sho 54-78982, Hei 6-77361, Hei 6-21278, and Hei 5-160306.
However, semiconductor integrated circuit elements, a substrate on which those semiconductor integrated circuit elements are mounted, and a cooling member have warping, uneven thickness, and roughness. Consequently, a solder-fixing method used with a multi-chip module causes a variability of solder thickness among solder portions which are soldered between the semiconductor integrated circuit elements and the cooling member.
In order to eliminate this variability, the following two methods have been considered: (1) to provide a proper amount of solder according to the necessary thickness at every soldered portion; and (2) to provide a fixed amount of solder and to dispose of surplus solder at every soldered portion.
In case of the method (1), an additional process is required to measure the thickness of solder at each soldered portion. Consequently, this method makes processing more complicated.
In case of the method (2), this method doesn't need to measure the thickness of the solder Technologies for disposing of surplus solder are disclosed in the Unexamined Published Japanese Patent Applications Nos. Sho 54-78982, Hei 6-21278, and Hei 5-160306. Especially, these documents disclose technology for simplifying the process of disposing of surplus solder. According to these technologies, a hole is formed corresponding to each of heat generating parts for disposing of surplus solder. The cooling member is soldered fixedly at the back side of heat generating parts such as semiconductor integrated circuit elements. In this case, solder is injected into the hole for filling and connecting the portions between the heat generating part and the cooling member with solder. Besides, if the surplus solder occurs, the surplus solder is disposed of by this hole for eliminating the variability of solder thickness at each soldered portion.
The above conventional technologies can dispose of surplus solder generated by a variability of solder thickness at soldered portions. But these technologies have the following problems.
In these technologies, the surplus solder is filled into a hole at each soldered portion. The amounts of filled surplus solder are each of the soldering portions are different from each other. Consequently, the positions of the solder surfaces in those holes are different from each other. As shown in the Unexamined Published Japanese Patent Nos. Sho 54-78982 and Hei 5-160306, those conventional technologies are still effective just as in the case of dealing with a low heat generation, such as using an air-cooling method.
But in case of dealing with a high test generation such as using a water-cooling method, some arrangements for the effective heat transfer from different heights of the solder surface in the holes are required. For example, in the Unexamined Published Japanese Patent Application No. Hei 6-21278, a low viscosity heat transfer material is disposed between a heat transfer plate which includes holes for filling and disposing solder and a cooling structure in which cooling water flows so as to make them in contact with each other for transferring the heat effectively, e.g., a heat transfer plate.